Heretofore, carbon black having a low structure has been widely used in the steel coating rubber for tires and industrial rubber products for improving the resistance to fracture and used in a great amount to prevent easy deformation due to excessive softness in many cases. This always causes poor dispersion of carbon black in the rubber, and drawbacks arise in that sufficient resistance to cut growth is not obtained, the effect of improving the resistance to fracture is not obtained to the expected degree, and the heat buildup is increased.
A process for producing a natural rubber master batch for improving the workability, the reinforcing property and the abrasion resistance, which comprises a step of decomposing the amide bond in which the amide bond in natural rubber is decomposed and a step of mixing in which the latex obtained after the step of decomposing the amide bond is mixed with a slurry containing an inorganic filler such as carbon black, silica, alumina and calcium carbonate dispersed in water, is known (Patent Reference 1).
However, the process for producing a natural rubber master batch described above is conducted when the inorganic filler is mixed into natural rubber to improve the abrasion resistance of tread rubber of tires and rubber at the surface layer of conveyor belts but not to improve the resistance to fracture and resistance to cut growth of steel coating rubber.
Under the above circumstances, a rubber composition comprising a natural rubber wet master batch which can control the size distribution of carbon black within a prescribed range with stability, provides excellent dispersion of carbon black and can achieve the improvement in the resistance to fracture and the resistance to cut growth in steel coating rubber in the step of mixing a natural rubber latex with a slurry containing carbon black dispersed in water by improving the properties of carbon black used in the production of a natural rubber wet master batch, has been desired.
[Patent Reference 1] Japanese Patent Application Laid-Open No. 204-99625